Several kinds of heat transfer liquids are used in the industry. Water is applied everywhere where it can be used as water is cheap, is not harmful to the environment and health, is not inflammable, it has a moderate corrosive effect and its specific heat is high. A great disadvantage of its application is that its volume increases by about 9% upon freezing and damages the instruments. From the point of view of power plants requiring a great cooling capacity a further disadvantage, which is of greater importance in regions being deficient in water, is that it has a quite low boiling point; therefore it has a relatively high tension. Therefore the evaporation rate is high and consequently the undesired loss of water can be significant.
The relatively low boiling point can be also disadvantageous when as a result of the boiling of water, e.g. in solar collectors, in the cooling system of vehicles, serious operating trouble occurs. Therefore it is desirable to raise the temperature range in which the liquid can be applied, i.e. raise the boiling point.
A further disadvantage of water is that it cannot be used for isopiestic drying. Under the term "isopiestic drying" such a procedure is understood wherein the water content of a heat sensitive substance to be dried (e.g. corn, maize) is removed by the aid of a water absorptive liquid at ambient temperature. The substance to be dried and the liquid are kept in the same closed drying space, wherein the air is circulated by the aid of a ventilator. The water absorbed by the liquid is removed by the batch or continuous distillation of the liquid and the liquid is periodically or continously recycled to the drying space. Thus not only the undesired heating of the substance to be dried can be avoided, but a great amount of energy can also be saved compared to the technique of drying with warm air.
The freezing point can be decreased and the boiling point can be increased by mixing well-soluble compounds in water. These effects, the so-called cryoscopic effects can be multiplied if strong electrolytes are dissolved in water as their electrolytic dissociation in water results in an increase of the apparent concentration even in concentrated solutions. A widely used, well-soluble strong electrolyte dissociating into three ions is calcium chloride. A drawback of its use is that similar to the other chloride compounds, it quickly corrodes the metal structural elements especially if the operation temperature is periodically or continously high. A special disadvantage of the chloride ions is that they cause local corrosion (hole, slit or intercrystalline corrosion), therefore the instrument can become worn out in certain cases even if its wall is substantially intact. Similar problems arise when other chloride compounds are used, while the application of acidic and basic solutions is limited by health and environment protective prescriptions besides the corrosion problems.
The lower alcohols, ketones, hydrocarbons (e.g. petrol) and cyclohexanone are preferred from the point of view of their anti-freeze character, but they are inflammable, their vapors constitute a volatile mixture with water, most of them are toxic and have a low boiling point. Simultaneously, the compounds boiling at a lower temperature than water are inapt for isopiestic drying. A further drawback of the listed solvents is that their thermal capacity related to a mass unit (specific heat) is small (abot 30 to 40% of the specific heat of water). As their density is less than 1, their specific heat related to a mass unit is even smaller. This latter fact has practical importance because the heat transfer liquids are used in instruments of predetermined volume.
The aqueous ethylene glycol solution is widely used in practice as a heat transfer liquid due to its several advantages. Glycol is miscible with water at any rates, moderately inflammable and explosive, its specific heat is about two times higher than that of the listed organic compounds, colorless and relatively resistant to freezing. The specific heat the freezing point depend on the ratio of glycol and water, the former one increases, while the latter one decreases with the decrease of the water content. The lowest freezing point belongs to the eutectic composition which comprises 60% of glycol. A further advantage is that the volume of the glycol/water mixture does not increase upon freezing, therefore the instrument is not damaged.
However, the above-listed advantages are accompanied with some technical drawbacks which restrict the further usability of the glycol/water mixture even if the price of glycol is not considered. Ethylene glycol is highly toxic and as it can be easily confounded with ethanol, lethal intoxications have occurred. Due to its toxicity it pollutes the environment, therefore cannot be led to the waste water. On the other hand, its use is restricted by the fact that the consistency of the 1:1 mixture being important for practical reasons is nonthixothropically pulpy even at -31.degree. C. (though the freezing point of this mixture is -38.degree. C., therefore it cannot be pumped. Thus not only the freezing point of a mixture has to be taken into consideration but the relatively narrow temperature range wherein the mixture gains a non-thixotropic viscous consistency, further referred to as fluidity limit, is also of great importance from a practical points of view. The fluidity limit at -31.degree. C. cannot be changed by varying the glycol/water ration, therefore the glycol/water mixture cannot be used for heat transfer below -31.degree. C. The anti-freeze character and fluidity parameters of the mixture decline with the progress of time. As a result of the slow polymerization of glycol, the mixture has to be replaced by a fresh one, which is a further technological-economy drawback.
We targeted our efforts for working out a novel heat transfer liquid meeting the following demands:
its boiling point should be higher than that of water, PA1 its specific heat should be similar to that of water, PA1 its freezing point should be varied by changing the compositions, its fluidity limit should be decreased below -31.degree. C. in case of necessity, PA1 its volume should not increase upon freezing, PA1 it should be miscible with water at any ratio, PA1 it should not be toxic, explosive and inflammable, it should not damage the environment, PA1 it should be miscible with the glycol/water mixture, PA1 it should be inodorous (to avoid air pollution), PA1 it should not be corrosive and its corrosive effect should not be greater than that of glycol/water mixture.